MOSCOW. (Alexander Bagrov, for RIA Novosti) - Almost a century ago, on June 17 (30), 1908, a massive explosion occurred near the Podkamennaya Tunguska River, in what is now Russia's Krasnoyarsk Territory, Central Siberia.

The residents of the Vanavara trading post, 65 kilometers (40 miles) south of the blast site, later claimed that the ground trembled violently when attacked by a huge ball of fire, followed by a terrible storm that destroyed everything in its wake.

The explosion was most likely caused by the airburst of a large meteoroid or comet fragment at an altitude of 5-10 kilometers (3-6 miles) from the Earth's surface. Studies have yielded varying estimates for the object's size, with most experts agreeing that it measured several dozen meters in diameter.

Estimates of the energy of the blast range from 5 megatons to as high as 30 megatons of TNT, with 10-15 megatons being the most likely yield. The blast, about 1,000 times more powerful than the Hiroshima bomb, felled an estimated 80 million trees over 2,150 square kilkometers (830 square miles). The earthquake caused by the blast measured 5.0 on the Richter scale. The region has never completely recovered.

The Tunguska blast was the largest meteoroid impact in the Earth's recent history, and demonstrated the awesome destructive power of near-space objects. An explosion of the scale of the one in Tunguska could destroy large metropolitan areas. It is this possibility that has helped to spark discussion of asteroid deflection strategies.

Due to the rotation of Earth, if the collision had occurred 4 hours 47 minutes later, it would have completely destroyed the Imperial Russian capital, St. Petersburg. A little later still, and the Tunguska meteorite would have wreaked chaos and destruction in densely populated Europe. Although scientists have advanced over 80 theories explaining the Tunguska event, none of them offers any conclusive evidence. Moreover, it is now impossible to verify them.

When I became an astronomer 30 years ago, I believed that the mystery of the Tunguska meteorite would never be solved. However, astronomers have since then obtained additional information about the origin of celestial bodies, and can offer more convincing explanations for the Tunguska blast.

I am going to tell an upcoming international conference in Moscow to mark the 100th anniversary of the Tunguska event that it was most likely caused by a comet fragment consisting of the Solar System's primary matter.

There are two kinds of comets in our Solar System. "Primary" comets consist of micron-sized inter-stellar dust and gas, whereas "secondary" comets feature meteorite substance. A disintegrating "secondary" comet forms the meteor showers that are frequently observed from the Earth. Some of their fragments do not burn up during reentry and can be recovered. On February 12, 1947, a large meteorite disintegrated spilled fragments over a 1.3-sq. km. area in the Sikhote-Alin range, some 440 km from Vladivostok in the Russian Far East. Subsequent expeditions recovered many iron fragments there.

The Astronomy Institute's experts and I believe that if the Tunguska meteorite were a "secondary" comet, then it would have contained several metric tons of meteor substance, and some of it could have been recovered.

It would be therefore logical to assume that the Tunguska meteorite was a huge gas-and-dust snowball, whose tiny fragments vaporized after hitting the terrestrial atmosphere. The remaining comet-nucleus also vaporized instantly, causing the loud and powerful air burst that was registered by many observatories all over the world.

The Tunguska meteorite was a milestone in terrestrial history, highlighting the dangers posed by killer asteroids. The other two known large meteorites that fell in the sparsely populated Sikhote-Alin range on February 12, 1947 and on August 13, 1930 in the Brazilian Amazonas region (approximately latitude 5Â° S. and longitude 71.5Â° W.), near the Peruvian frontier, caused no substantial damage.

Such narrow escapes are either a coincidence, or God's Providence. At the same time, we know very little about meteorites falling into the world's oceans.

With the onset of the Space Age, humankind realized that the Earth was under constant threat from celestial bodies. Asteroid-and-comet studies have soared in popularity as people have become aware that these space objects have the capability to wipe out human civilization. Even now, numerous near-Earth satellites designed to detect ballistic-missile launches or unauthorized nuclear tests annually register 10-15 airbursts equivalent to a megaton of TNT. Most scientists believe that these explosions are the result of mini-comets burning up in the atmosphere.

Although we still lack the technology to deflect approaching asteroids and comets, we can watch the skies and evacuate the population of threatened areas in case of an impending strike.

Hollywood films notwithstanding, it would be unforgivably dangerous to use weapons to destroy an incoming asteroid. The resulting fragments could cause at least as much damage as the original object. Probably the best solution would be to land a small rocket on the asteroid's surface, exerting just enough force to gently change its trajectory. This would be far less spectacular, but could save thousands of lives, as well as humankind's material and spiritual heritage.

Doctor of Physics and Mathematics, Alexander Bagrov, is a leading research associate of the Astronomy Institute of the Russian Academy of Sciences.

The opinions expressed in this article are the author's and do not necessarily represent those of RIA Novosti.